@Olivier5,
Any person who is arranging a date for casual sex already understands perfectly well that they are entitled to enjoy a "good shot" and nothing more. That is the very idea of casual sex.
Pretexts are a bad thing when it comes to sexual trysts, in this type of arrangement it is important to be up front so that both people involved understand perfectly well what it means.
Say clearly what you want and how you want, before you turn on the neuro-chemical pumps. If you don't want to see my ugly face ever again after we have coitus... just tell me, I can take it.
Telling me you need to go grocery shopping is just plain mean.